1. Technical Field
This disclosure relates generally to a spark plug for internal combustion engines which has an annular ground electrode disposed to face an outer circumference of a center electrode, and a production method thereof.
2. Background Art
For instance, Japanese Patent No. 5075127 discloses a spark plug for use in internal combustion engines mounted in automotive vehicles or cogeneration systems. The spark plug has an annular ground electrode which faces an outer periphery of a center electrode. The joint of the ground electrode to a housing of the spark plug is achieved by crimping a front end of the housing inwardly to define a spark gap between the outer circumference of the center electrode and the inner circumference of the annular ground electrode.
The above spark plug, as described above, has the structure in which the ground electrode is tightly held inside the crimped front end of the housing to make a mechanical contact of the outer periphery of the ground electrode with the housing, thus resulting in an increased length of a heat dissipating path between the inner peripheral surface of the ground electrode which faces the spark gap and the housing. This increases the risk of higher temperatures occurring in the ground electrode, which usually leads to an increase in mechanical wear of portions of the electrodes around the spark gap, thereby accelerating the rate at which the size of the spark gap increases, thereby shortening the time taken to reach an upper limit of a size of the spark gap. It is, thus, difficult to produce spark plugs which have an increased service life.
The ground electrode is disposed inside the housing. The spark gap has an end located inside the front end of the housing in the lengthwise direction of the spark plug, thus encountering a probability that it is difficult for flame, as created by a spark generated in the spark gap, to grow, that is, a cooling loss increases in the spark plug, which will result in a reduced ability to ignite fuel in the engine.
The above spark plug, as described above, has the ground electrode tightly pressed inside the housing. It is, thus, difficulty to adjust the position of the ground electrode, that is, the size of the spark gap. Specifically, the accurate creation of the spark gap between the outer periphery of the center electrode and the inner periphery of the ground electrode requires increased accuracy in positioning the ground electrode relative to the center electrode. A variation in dimension or an assembling error of parts such as the housing etc. results in a failure in forming a desired spark gap even if the ground electrode is accurately positioned relative to the housing, thus requiring the need to position the ground electrode relative to the center electrode. However, in the structure in which the ground electrode is disposed radially inside the crimped front end of the housing, a great variation in dimension or a great assembling error of parts of the spark plug will restrict movement of the ground electrode in a radial direction of the spark plug. It is, thus, difficult to accurately adjust the size of the spark gap.